//6.N字形变换
class Solution {
    public String convert(String s, int numRows) {
        //处理边界情况
        if(numRows == 1){
            return s;
        }
        int d = 2 * numRows - 2;
        int n = s.length();
        StringBuilder ret = new StringBuilder();
        //1.处理第一行
        for(int i = 0; i < n;i += d){
            ret.append(s.charAt(i));
        } 

        //2.处理中间行
        for(int k = 1;k < numRows - 1;k++){
            for(int i = k,j = d-i;i < n || j < n;i += d,j += d){
                if(i < n){
                    ret.append(s.charAt(i));
                }
                if(j < n){
                    ret.append(s.charAt(j));
                }
            }
        }

        //3.处理最后一行
        for(int i = numRows - 1;i < n;i += d){
            ret.append(s.charAt(i));
        }

        return ret.toString();
    }
}